structure and function of hemoglobin
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Structure and function of hemoglobin. Dr. Sumbul Fatma Clinical Chemistry Department of Pathology. Hemoglobin (Hb). A hemeprotein found only in red blood cells Oxygen transport function Contains heme as a prosthetic group Heme reversibly binds to oxygen. The heme group. - PowerPoint PPT PresentationTRANSCRIPT
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Structure and function of hemoglobin
Dr. Sumbul FatmaClinical Chemistry
Department of Pathology
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Hemoglobin (Hb)
• A hemeprotein found only in red blood cells• Oxygen transport function• Contains heme as a prosthetic group• Heme reversibly binds to oxygen
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The heme group
• A complex of protoporphyrin IX and ferrous iron (Fe2+)
• Fe2+ is present in the center of heme• Binds to four nitrogens of the porphyrin ring• Plus two additional bonds with:
– Histidine residue of globin chain– Oxygen
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The heme group: Fe2+– porphyrin complex with bound O2
Pag
e 32
5
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Types of Hb
Normal: HbA (97%)HbA2 (2%)
HbF (1%)HbA1c
Abnormal: Carboxy HbMet HbSulf Hb
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Hemoglobin A (HbA)
• Major Hb in adults• Composed of four polypetide chains:
– Two α and two β chains
• Contains two dimers of αβ subunits• Held together by noncovalent interactions• Each chain is a subunit with a heme group in the
center that carries oxygen• A Hb molecule contains 4 heme groups and
carries 4 moelcules of O2
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HbA structure
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T-form of Hb
• The deoxy form of Hb• Taut form• The movement of dimers is
constrained• Low oxygen affinity form
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R-form of Hb
• The oxygenated form of Hb• Relaxed form• The dimers have more
freedom of movement• High-oxygen-affinity form
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Hemoglobin function
• Carries oxygen from the lungs to tissues• Carries carbon dioxide from tissues back to
the lungs• Normal level:
• Males: 14-16 g/dL• Females: 13-15 g/dL
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Factors affecting oxygen binding
• Three allosteric effectors:– pO2 (partial oxygen pressure)
– pH of the environment– pCO2 (partial carbon dioxide pressure)
– Availability of 2,3-bisphosphoglycerate
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Oxygen Dissociation Curve
• The curve is sigmoidal• Indicates cooperation of
subunits in O2 binding
• Binding of O2 to one heme group increases O2 affinity of others
• Heme-heme interaction
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P50 (Partial oxygen pressure)– P50 (mm Hg): the pressure at which Hb is 50%
saturated with O2
– Indicates affinity of Hb to O2
– High affinity slow unloading of O2
– Low affinity fast unloading of O2
– Lung pO2 is 100 mm Hb saturation 100%– Tissue pO2 is 40 mm Hb saturation reduces to
75%– Oxygen delivery to tissues 25%
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The Bohr effect• Effect of pH and pCO2 on:
– Oxygenation of Hb in the lungs– Deoxygenation at the tissues
• Tissues have lower pH (acidic) than lungs
• Due to proton generatation:CO2 + H20 ------> HCO3
- + H+
• Protons reduce O2 affinity of Hb
• Causing easier O2 release into the tissues
• The free Hb binds to two protons
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The Bohr Effect • Protons are released and react with HCO3 – to form CO2 gas
• The proton-poor Hb now has greater affinity for O2
• The Bohr effect removes insoluble CO2 from blood stream• Produces soluble bicarbonate
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Availability of 2,3-bisphosphoglycerate
• Binds to deoxy-hb and stabilizes the T-form• When oxygen binds to Hb, BPG is released
At high altitudes there is-increase in no. of RBcs-Increase in conc. Of Hb-Increase in BPG
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High altitude and O2 affinity
• High altitude decreases Hb O2 affinity• Hypoxia
– Increases 2,3 DPG levels– Decreases O2 affinity– Increases O2 delivery to tissues
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High O2 affinity
• High O2 affinity occurs due to:• Alkalosis• High levels of abnormal Hb (met Hb, CO-Hb)• High levels of Hb F• Multiple transfusion of 2,3 DPG-depleted
blood
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Fetal Hemoglobin (HbF)• Major hemoglobin found in the fetus and newborn• Tetramer with two α and two chains• Higher affinity for O2 than HBA• Transfers O2 from maternal to fetal circulation
across placenta
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HbA2
• Appears ~12 weeks after birth• Constitutes ~2% of total Hb• Composed of two α and two δ globin chains
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HbA1c
• HbA is slowly and non-enzymatically glycosylated
• Glycosylation depends on plasma glucose levels
• HbA1c levels are high in patients with diabetes mellitus
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Abnormal Hbs
• Unable to transport O2 due to abnormal structure
• Carboxy-Hb: CO replaces O2 and binds 200X tighter than O2(in smokers)
• Met-Hb: Contains oxidized Fe3+ (~2%) that cannot carry O2
• Sulf-HB: Forms due to high sulfur levels in blood (irreversible reaction)